The dose response of high‐resolution diode‐type detectors and the role of their structural components in strong magnetic field

Tekin, Tuba; Blum, Isabel; Delfs, Björn; Schönfeld, Ann-Britt; Kapsch, Ralf Peter; Poppe, Björn; Looe, Hui Khee

doi:10.1002/mp.14535

Cited by 8 publications

(21 citation statements)

References 18 publications

(49 reference statements)

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“…Table 1: Diameters of the detectors including wall material and of the detectors sensitive area for all investigated point detectors as well as the sensitive detector volumes in mm 3 as calculated from finite-element methods taken from (Tekin et al, 2020) (*) or calculated using data given in (Tekin et al, 2022) The detector specific K(x) were derived according to Equation 1 from the measured D(x) and M(x) using the iterative van Cittert (1931) deconvolution method as described in Looe et al (2015). The number of iterations was limited to five to suppress the unavoidable noise amplification during the iteration process.…”

Section: Signal Profiles M(x)mentioning

confidence: 99%

Investigating the lateral dose response functions of point detectors in proton beams

Kretschmer¹,

Brodbek²,

Looe³

et al. 2022

Phys. Med. Biol.

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Objective Point detector measurements in proton fields are perturbed by the volume effect originating from geometrical volume-averaging within the extended detector’s sensitive volume and density perturbations by non-water equivalent detector components. Detector specific lateral dose response functions K(x) can be used to characterize the volume effect within the framework of a mathematical convolution model, where K(x) is the convolution kernel transforming the true dose profile D(x) into the measured signal profile of a detector M(x). The aim of this work is to investigate K(x) for detectors in proton beams. Approach The K(x) for five detectors were determined by iterative deconvolution of measurements of D(x) and M(x) profiles at 2 cm water equivalent depth of a narrow 150 MeV proton beam. Monte Carlo simulations were carried out for two selected detectors to investigate a potential energy dependence, and to study the contribution of volume-averaging and density perturbation to the volume effect. Main results The Monte Carlo simulated and experimentally determined K(x) agree within 2.1% of the maximum value. Further simulations demonstrate that the main contribution to the volume effect is volume-averaging. The results indicate that an energy or depth dependence of K(x) is almost negligible in proton beams. While the signal reduction from a Semiflex 3D ionization chamber in the center of a gaussian shaped field with 2 mm sigma is 32% for photons, it is 15% for protons. When measuring the field with a microDiamond the trend is less pronounced and reversed with a signal reduction for protons of 3.9% and photons of 1.9%. Significance The determined K(x) can be applied to characterize the influence of the volume effect on detectors measured signal profiles at all clinical proton energies and measurement depths. The functions can be used to derive the actual dose distribution from point detector measurements.

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Section: Signal Profiles M(x)mentioning

confidence: 99%

Investigating the lateral dose response functions of point detectors in proton beams

Kretschmer¹,

Brodbek²,

Looe³

et al. 2022

Phys. Med. Biol.

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“…For a perpendicular MRI-linac, the magnetic field correction factor includes a correction for the difference in dose deposition of 0.995 [8,9]. Tekin et al [26] has shown that the k B,Q for the microDiamond is approximately 1 when irradiated in "edge-on" orientation, i.e., the long axis of the microDiamond is aligned to the B 0 field direction and perpendicular to the radiation axis. The simulated correction factor for the microDiamond incorporates the correction for the difference in the dose deposition.…”

Section: Theorymentioning

confidence: 99%

“…Our work investigated the microDiamond as a crosscalibration detector for determination of ion chamber magnetic field correction factors via the ratio of the microDiamond and an ion chamber measured on both a conventional linac and a MRI-linac. The microDiamond was investigated due to the small energy dependence [25] and small magnetic field correction factor [26] associated with the detector.…”

Section: Introductionmentioning

confidence: 99%

Ion chamber magnetic field correction factors measured via microDiamond cross-calibration from a conventional linac to MRI-linac

et al. 2022

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Magnetic field correction factors are required for performing reference dosimetry on Magnetic Resonance Imaging Linear accelerators (MRI-linacs). Methods for measuring magnetic field correction factors usually require specialized equipment and expertise. Our work investigated the use of a microDiamond detector to cross-calibrate an ion chamber between a conventional linac and MRI-linac as a method to measure ion chamber magnetic field correction factors for the MRI-linac. Ratios of the microDiamond and ion chamber were measured on a conventional linac, parallel MRI-linac at 0 T, parallel MRI-linac at 1 T and perpendicular MRI-linac at 1.5 T. The beam quality dependence of the microDiamond was investigated by comparing the measurements on the conventional linac and parallel MRI-linac at 0 T. The magnetic field dependence of the microDiamond was investigated comparing the measurements on a parallel MRI-linac at 0 and 1 T. The ion chamber magnetic field correction factors were calculated by comparing the conventional linac and parallel MRI-linac at 1 T and the conventional linac and perpendicular MRI-linac at 1.5 T for the parallel and perpendicular factors respectively. The FC65-G and PTW30013 ion chambers were investigated. For a parallel MRI-linac, with a beam quality of TPR20,10 = 0.632, we measured magnetic field correction factors of 0.988 ± 0.016 (k = 2) and 0.987 ± 0.016 (k = 2) for a FC65-G and PTW30013 respectively, where k is the coverage factor. For a perpendicular MRI-linac, with a beam quality of TPR20,10 = 0.701, we measured magnetic field correction factors of 0.995 ± 0.020 (k = 2) and 0.983 ± 0.020 (k = 2) for a FC65-G and PTW30013 respectively. The results showed agreement with previously published work which used different approaches. Our work demonstrates the use of a microDiamond to calculate the ion chamber magnetic field correction factor using measurements on a conventional linac and MRI-linac. The high level of uncertainty in our results means the method at present can only be used for validation of magnetic field correction factors.

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“…Correction factors have been derived to account for this, and depends on the magnetic field strength and directions 26–31 . For solid state‐based detectors, there is no apparent changes within the bulk of the detection volume, rather the diode packaging and any air‐gaps surrounding the diodes generate changes in the secondary electron fluence 32–35 …”

Section: Introductionmentioning

confidence: 99%

“…[26][27][28][29][30][31] For solid state-based detectors, there is no apparent changes within the bulk of the detection volume, rather the diode packaging and any air-gaps surrounding the diodes generate changes in the secondary electron fluence. [32][33][34][35] The purpose of the current work is to present the successful workings of a portable device capable of producing strong magnetic fields over a volume large enough to support fundamental small-scale experiments that mimic the environment of an MRI-linac system. The device is given the acronym MARDOS: Magnetic Apparatus for RaDiation Oncology Studies.…”

Section: Introductionmentioning

confidence: 99%

A portable magnet for radiation biology and dosimetry studies in magnetic fields

et al. 2022

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Background and Purpose In the current and rapidly evolving era of real‐time MRI‐guided radiotherapy, our radiation biology and dosimetry knowledge is being tested in a novel way. This paper presents the successful design and implementation of a portable device used to generate strong localized magnetic fields. These are ideally suited for small‐scale experiments that mimic the magnetic field environment inside an MRI‐linac system, or more broadly MRI‐guided particle therapy as well. Materials and Methods A portable permanent magnet‐based device employing an adjustable steel yoke and magnetic field focusing cones has been designed, constructed, and tested. The apparatus utilizes two banks of Nd2$_{2}$Fe14$_{14}$B permanent magnets totaling around 50 kg in mass to generate a strong magnetic field throughout a small volume between two pole tips. The yoke design allows adjustment of the pole tip gap and exchanging of the focusing cones. Further to this, beam portal holes are present in the yoke and focusing cones, allowing for radiation beams of up to 5 ×$\times$ 5 cm2$^{2}$ to pass through the region of high magnetic field between the focusing cone tips. Finite element magnetic modeling was performed to design and characterize the performance of the device. Automated physical measurements of the magnetic field components at various locations were measured to confirm the performance. The adjustable pole gap and interchangeable cones allows rapid changing of the experimental set‐up to allow different styles of measurements to be performed. Results A mostly uniform magnetic field of 1.2 T can be achieved over a volume of at least 3 ×$\times$ 3 ×$\times$ 3 cm3$^{3}$. This can be reduced in strength to 0.3 T but increased in volume to 10 ×$\times$ 10 ×$\times$ 10 cm3$^{3}$ via removal of the cone tips and/or adjustment of the steel yoke. Although small, these volumes are sufficient to house radiation detectors, cell culture dishes, and various phantom arrangements targeted at examining small radiation field dosimetry inside magnetic field strengths that can be changed with ease. Most important is the ability to align the magnetic field both perpendicular to, or inline with, the radiation beam. To date, the system has been successfully used to conduct published research in the areas of radiation detector performance, lung phantom dosimetry, and how small clinical electron beams behave in these strong magnetic fields. Conclusions A portable, relatively inexpensive, and simple to operate device has successfully been constructed and used for performing radiation oncology studies around the theme of MRI‐guided radiotherapy. This can be in either inline and perpendicular magnetic fields of up to 1.2 T with x‐ray and particle beams.

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The dose response of high‐resolution diode‐type detectors and the role of their structural components in strong magnetic field

Abstract: The dose response of high-resolution diode-type detectors and the role of their structural components in strong magnetic field

Cited by 8 publications

References 18 publications

Investigating the lateral dose response functions of point detectors in proton beams

Investigating the lateral dose response functions of point detectors in proton beams

Ion chamber magnetic field correction factors measured via microDiamond cross-calibration from a conventional linac to MRI-linac

A portable magnet for radiation biology and dosimetry studies in magnetic fields

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